1. Field of Invention
The present invention relates to an aluminum-alloy starting powder for producing a sintered aluminum-alloy, a sintered aluminum-alloy, and a method for producing the sintered aluminum-alloy.
2. Description of Related Arts
Recently, demands for light aluminum-alloy parts are increasing in the field of office machines and computer-related machinery and equipment, because power consumption must be lessened, noise generation due to vibration must be prevented, and, further, portability of the machines must be improved.
One of the methods for producing the aluminum-alloy parts is the ordinary powder metallurgy method, which comprises a pressing and sintering process. The products of the powder metallurgy are greatly advantageous over die castings and wrought products, in the fact that precise parts having near net shape and free of defects can be produced by a simple process.
The compositions of the sintered aluminum-alloy are usually similar to or belong to 2000 series or 6000 series of AA standard, which are heat treatable and hence can exhibit a high strength level (c.f. J. D. Generous and W. C. Montgomery, Chapter 8 "Aluminum P/M Properties and Applications" Powder Metallurgy, Edited by E. Klar, P211-234, and ASTM Designation: B595-84 Standard Specification for SINTERED ALUMINUM ALLOY STRUCTURAL PARTS).
The so-called blended elemental method is well known for producing the aluminum-alloy precision parts by the pressing and sintering process. The starting powder used in the blended elemental method is a mixture of pure Al powder and elemental powder of such alloying elements as Cu, Si, Mg, and the like which form a low-melting point eutectic with Al. However, the elemental powder has a high melting point, and, further, the mean distance between the particles of the elemental powder is great in the green compact. Uniform diffusion of the elements and satisfactory formation of the eutectic occur with difficulty. In addition, the alloying elements may remain unalloyed in the sintered product. The blended elemental method therefore results in a sintered aluminum-alloy with a high strength being produced with difficulty.
There is a pre-alloy method for producing aluminum-alloy precision parts by the pressing and sintering process. According to this method, one or more alloying elements is preliminarily added to the Al powder so as to provide the starting powder having the final composition, i.e., the composition of the sintered product. In this method, the alloying element(s) so hardens the starting powder that it is difficult to shape the starting powder by pressing. A green compact therefore has very low density. In addition, since the alloying element(s) lowers the melting point of the starting powder, it therefore becomes difficult to enhance the sintering temperature so as to cause satisfactory diffusion and sintering. Furthermore, since the melting point of all the particles of the starting powder is identical, the liquid phase is not formed in the proper amount but is formed either excessively or very low.
There is a master-alloy method for producing the aluminum-alloy precision parts by the pressing and sintering process (c.f. for example Japanese Unexamined Patent Publication No. 1-294833). According to this method, one or more alloying elements is added to Al powder to prepare the master-alloy. The master-alloy is mixed with pure Al powder to prepare a starting-mixture powder. The composition of the master-alloy is so adjusted that a multi-system eutectic having a low melting point is easily formed during the sintering.
There are demands for a method for producing a sintered aluminum-alloy having a high strength and elongation.
When sintered parts having a complicated shape are produced by the conventional methods, they exhibit poor mechanical properties, particularly poor elongation. The mixture alloy-powder and pure aluminum-powder are difficult to uniformly compact to density in a green compact into a die and to uniformly shape by compressing. The sintered aluminum-alloy has therefore locally low density, which causes reduction in the mechanical strength of the sintered aluminum-alloy.